Detection of hot gas in the filament connecting the clusters of galaxies Abell 222 and Abell 223

About half of the baryons in the local Universe are invisible and - according to simulations - their dominant fraction resides in filaments connecting clusters of galaxies in the form of low density gas with temperatures in the range of 10^5<T<10^7 K. The existence of this warm-hot intergalactic medium was never unambiguously proven observationally in X-rays. We probe the low gas densities expected in the large scale structure filaments by observing a filament connecting the massive clusters of galaxies A 222 and A 223 (z=0.21) which has a favorable orientation approximately along our line of sight. This filament has been previously detected using weak lensing data and as an over-density of colour selected galaxies. We analyse X-ray images and spectra obtained in a deep observation (144 ks) of A 222/223 with XMM-Newton. We present here observational evidence of the X-ray emission from the filament connecting the two clusters. We detect the filament in the wavelet decomposed soft band (0.5-2.0 keV) X-ray image with a 5 sigma significance. Following the emission down to 3 sigma significance, the observed filament is approximately 1.2 Mpc wide. The temperature of the gas associated with the filament determined from the spectra is kT=0.91\pm0.25 keV and its emission measure corresponds to a baryon density of (3.4\pm1.3)x10^-5 (l/15Mpc)^-1/2 cm^-3, where l is the length of the filament along the line of sight. This density corresponds to a baryon over-density of approximately 150. The properties of the gas in the filament are consistent with the results of simulations for the densest and hottest parts of the warm-hot intergalactic medium.